Source: http://lifehacker.com/5583406/ad+aware-adds-antivirus-scheduled-scans-to-free-version
It used to be that the free version of Ad-Aware was crippled in order to make you pay for a full license, but in an age of lots of great free antivirus and anti-spyware software, it seems Ad-Aware has taken the hint and added these fairly basic features to the free version. Before, if you wanted the free version, you'd need a separate antivirus program and you'd need to start your malware scans manually—not exactly ideal. However, among all their other recent additions, Ad-Aware has put their scheduler and Sunbelt-powered Antivirus scanner in the free version, likely hoping to compete with the other popular, free security suites out there. Hit the link to check it out, and let us know what you think of all the latest improvements in the comments.
Ad-Aware is a free download for Windows, though also has Plus and Pro versions available for $30 and $50, respectively.